Is there any reason they couldn't also apply export-control to older models, just to screw with Anthropic?

Every. There is no reason the government will let go the power it has obtained, that is never how it works

There won't be any new generation of models more powerful than Fable since the argument against Fable would apply even more. Opus 4.8 and GPT-5.5 is the best we'll ever see from this point forward. Soon low cost Chinese models will catch up to those thereby destroying Anthropic and OpenAI's pricing power which will mark the beginning of the end for them too.